German investigators have been unable to determine the precise cause of an uncontained auxiliary power unit fire which destroyed an Antonov An-12 freighter preparing for departure from Leipzig.
But doubts have emerged over the maintenance record of the APU at the time of the accident on 9 August 2013.
Investigation authority BFU says the crew had started the APU and both outboard engines of the Ukraine Air Alliance transport, which was parked on stand 207, when they were alerted by a “dull bang” and the APU fire-warning indicator.
The APU was mounted behind the left main landing-gear. BFU’s inquiry found the fire originated in the APU and was not contained, propagating rapidly to the cargo compartment.
It says the severity of the fire was exacerbated by fuel, which had leaked from fuselage-floor tanks and pooled near the left main landing-gear, as well as burning light metal alloy components from the APU.
The fuselage forward of the empennage was completely consumed by the blaze.
“Due to the high degree of destruction it was not possible to determine the exact cause of the fire,” says BFU.
But it reconstructed the APU gas turbine and its gearbox assembly and says there is evidence of a burst compressor wheel, given the nature of damage to air intakes and other components.
“Flying fragments of the compressor wheel could have penetrated the APU chamber and severed fuel pipes,” the inquiry adds. The damage would have provided a propagation path to the cargo compartment, which was loaded with nearly 49,000 day-old chicks.
The cargo door had been left open, because of the live cargo, and the inquiry suggests the blaze would have been strengthened by the available oxygen.
Although the crew had activated the APU extinguisher, the extent of the damage from the initial event would explain why this was ineffective in controlling the fire.
It states that the area of the APU fire was “not sufficiently isolated” from the rest of the aircraft (UR-CAG).
Investigators state that the APU, built in 1975, had a total operating time of 407h and, according to the operator, had undergone three overhauls in 1983, 1996 and 2007.
But Russia’s Interstate Aviation Committee says the APU manufacturer, Aviamotor, had stopped maintenance of the equipment around 2000, and there is “some doubt” that the claimed 2007 overhaul took place. In formal comments to the inquiry, it says it believes the logbook entry on this overhaul was “fabricated”.
It adds that the “non-fulfilment” of this overhaul could have contributed to the APU’s extensive damage, although BFU says it is unable to determine whether this was a factor. The APU was fitted to the aircraft in 2012.
BFU says the flight-data recorder tape had been inserted the wrong way, although it had registered the An-12’s arrival at Leipzig, while recovery of the two cockpit-voice recorders revealed one was empty and the other contained an 11min recording which was not captured at Leipzig.